David Cameron launched an 11th-hour bid to fend off the threat from Ukip when he indicated that he was prepared to give serious consideration to a Tory backbench campaign to place an EU referendum on the statute book before the next election.

As the Tories brace themselves for losses of at least 500 seats in the English local elections on Thursday – with a strong showing by Ukip – the prime minister said he was prepared to "strengthen" his offer of an in/out referendum by 2017.

The prime minister issued the signals to the right as the Tories struggled to maintain a consistent line on the threat posed by Ukip. William Hague echoed remarks over the weekend by Kenneth Clarke who dismissed the party as a "collection of clowns", when he said the party had a "clown-like aspect". The prime minister, by contrast, declined to criticise Ukip and would not even use its name in a BBC interview.

Nigel Farage, the Ukip leader, joked that the Tory insults had helped his party. Farage told ITV News: "Keep the abuse coming, boys, keep it coming. Ken Clarke had a go at the weekend and we went up by 2% in the opinion polls afterwards. So please keep abusing us, because when you abuse us what you are doing is you are abusing the vast majority of the British public who want us to be a free independent country that controls its own borders."

The prime minister used his final interview ahead of local elections to reach out to voters tempted by Ukip when he indicated that he may be prepared to throw the weight of the 304 Tory MPs behind placing a referendum on the statute book before the next election.

The prime minister is facing strong pressure from the Tory right to face down the Liberal Democrats, who would oppose any legislation for a referendum in this parliament. John Baron, the MP for Basildon and Billericay, has won the support of 100 Tory MPs for a letter calling on the prime minister to bring forward enabling legislation for a referendum.

Cameron told The World at One on Radio 4: "I think we need to demonstrate absolutely that we are serious about this referendum. We have said we are going to hold it. It is going to be an in/out referendum. We have set a date by which it must be held.

"I look forward to publishing a bill, to getting support for it, to doing everything I can to show to people at the next election there will be a real choice. If you want a party that is going to reform the EU and Britain's place in it and then give you a proper in out choice there only is one option: that is the Conservative party. Anything we can do to strengthen that offer I am prepared to consider."

Downing Street sources acknowledged that the prime minister would not be able to introduce any government legislation in the face of Lib Dem opposition. But they indicated that Cameron may be prepared to support moves by backbenchers.

Baron says that the new session of parliament, which opens on 8 May, offers the chance when a ballot for private members' bill will be held. If an MP who comes near the top of the ballot supported a referendum bill it would have a strong chance if the Tory leadership lent its support.

Baron said: "Our commitment to a referendum must be both credible and believable. It's credible because we have the out option now. But it is not yet believable because there is a deep public mistrust out there when people hear politicians making promises about EU referenda. They have heard too many before and too many have been broken.

"What would make it more believable is a serious attempt to get legislation through in this parliament. Whether successful or not it really would show good intent and would be more believable than a general election manifesto promise. It would be in law."

The prime minister also reiterated his remarks during his trip to India in February that aid spending could be diverted to peacekeeping. He told the BBC: "There are rules for what you can use the aid budget for and it is right we obey those rules. But I also think it is right we ask the question about what does development actually consist of. Of course there are vaccination programmes...But there is an argument that one of the most fundamental parts of development is security...The rules already allow some of your aid money to be spent on security issues and we should make sure that within the rules we are doing what is necessary not to make the numbers add up but actually so that development takes place."

The Tories are defending nearly two-thirds (1,477) of the 2,362 seats up for grabs on Thursday. They expect heavy losses as they defend seats captured when they were at a polling high in 2009, and are nervous they will be squeezed by the anti-EU Ukip.